Ftp memory device with single selection transistor

ABSTRACT

An embodiment of a non-volatile memory device integrated in a chip of semiconductor material is proposed. The memory device includes a plurality of memory cells. Each memory cell includes a first well and a second well of a first type of conductivity that are formed in an insulating region of a second type of conductivity. The memory cell further includes a first, a second, and a third region of the second type of conductivity that are formed in the first well; these regions define a selection transistor of MOS type and a storage transistor of floating gate MOS type that are coupled in series. Moreover, the memory device includes a selection gate of the selection transistor, a floating gate of the storage transistor, and a control gate of the storage transistor formed in the second well; the control gate is capacitively coupled with the floating gate. An embodiment, the memory device further includes means for applying a first programming voltage to the first wells and a second programming voltage to the control gates of a subset of the memory cells including at least one memory cell to be programmed (with a difference between the first programming voltage and the second programming voltage that is capable of injecting electric charge into the floating gate of each memory cell to be programmed), for applying the first programming voltage to the third region of each memory cell to be programmed, and for applying a third programming voltage (3.5V) comprised between the first programming voltage and the second programming voltage to the third region of each memory cell of the subset not to be programmed (with a difference between the first programming voltage and the third programming voltage that inhibits the injection of electric charge into the corresponding floating gate).

PRIORITY CLAIM

The instant application claims priority to Italian Patent Application No. MI2009A002349, filed Dec. 30, 2009, which application is incorporated herein by reference in its entirety

RELATED APPLICATION DATA

This application is related to U.S. patent application Ser. No.: ______, entitled FTP MEMORY DEVICE WITH PROGRAMMING AND ERASING BASED ON FOWLER-NORDHEIM EFFECT (Attorney Docket No.: 2110-357-03) filed ______; and is related to U.S. patent application Ser. No.: ______, entitled FTP MEMORY DEVICE PROGRAMMABLE AND ERASABLE AT CELL LEVEL (Attorney Docket No.: 2110-364-03) filed ______, all of the foregoing applications are incorporated herein by reference in their entireties.

TECHNICAL FIELD

An embodiment relates to the electronics field. More specifically, an embodiment relates to non-volatile memory devices.

BACKGROUND

Non-volatile memory devices are used in applications requiring the storing of information that has to be retained even when the memory devices are not powered. Generally, each memory device includes a matrix of memory cells based on floating gate MOS storage transistors; each storage transistor has a threshold voltage that may be set (according to an electric charge in its floating gate) to different levels representing corresponding logic values.

For example, in Electrical Erasable and Programmable Read-Only Memories (E²PROMs) each storage transistor may be both programmed (by injecting electric charge into its floating gate) and erased (by removing electric charge from its floating gate) individually—thanks to the use of a set of MOS selection transistors that apply the required voltages selectively to the corresponding storage transistor (with a quite complex structure that limits the capacity of the E²PROMs to a few Kbytes). On the other hand, flash memories have a simple structure that allows obtaining very high capacities thereof, up to some Gbytes—thanks to the grouping of the memory cells in sectors each one integrated in a common well of semiconductor material without any selection transistor (with the need of erasing the flash memories at the sector level).

In both cases, a production process of the memory devices substantially differs from a standard one (for example, in CMOS-technology). Indeed, the storage transistors may require an additional polysilicon layer to define their floating gates (besides the one used to define their control gates as in the CMOSs). This difference may add design complexity, which may significantly increase the manufacturing cost of the memory devices (of the order of 30% with respect to standard CMOS devices).

In order to solve these problems, Few Time Programmable (FTP) or Cost-Effective memories have been proposed in the last years. In the FTP memories, the memory cells are again grouped in sectors (integrated in corresponding wells). However, the storage transistor of each memory cell now has a distinct control gate region being capacitively coupled with its floating gate; therefore, the FTP memories use a single polysilicon layer, so that they may be manufactured with the standard CMOS production process.

The FTP memories known in the art are generally based on a so-called emitter structure. Particularly, in the emitter-FTP memories, each memory cell includes, in addition to the storage transistor, an MOS selection transistor (being used to select the memory cell for its reading), and a stray BJT injection transistor (being used to implement its programming). The memory cells are programmed by hot electron injection (very fast through their injection transistors) and they are erased by Fowler-Nordheim effect. An example of emitter-FTP memory is described in U.S. Pat. No. 6,876,033 (the entire disclosure of which is herein incorporated by reference).

This makes the FTP memories very attractive for the embedding of memories of small capacity (up to some Kbytes) into CMOS devices; indeed, in this case it may possible to add the FTP memories at low cost and very quickly (since they do not require complex re-design and test operations).

However, FTP-memories may also have to be erased at the sector level. Moreover, they may require very high voltages (both positive and negative) for programming and erasing the memory cells, and high currents for their programming. Particularly, in the emitter-FTP memories the injection transistors have a stray structure; therefore, the injection transistors may not be characterized accurately, and thus they may have be over-dimensioned to ensure the required performance in every anticipating operating condition. Therefore, the currents that are used during the programming of the memory cells may further increase (up to 0.1-5 mA). This may require very complex charge pumps to generate the required high voltages (from a lower power supply voltage), and at the same time provide the required high currents. In addition, the high voltages and currents may involve large power consumption. These high currents may also limit a programming parallelism (for example, to 8 memory cells). Moreover, during the erasing of a selected sector, the high voltages applied to its memory cells may in part propagate to the other memory cells that are capacitively coupled thereto. As a result, these memory cells may be subject to an electric stress that causes a loss of electric charge in the floating gates of their storage transistors, with an undesired erasing of the memory cells after repeated erasing operations on nearby cells. This may limit a data retention of the memory cells, and then a number of erasing operations that may be withstood (of the order of a few thousands).

SUMMARY

An embodiment is based on the idea of implementing both the erasing and the programming by Fowler-Nordheim effect.

More specifically, an embodiment is a non-volatile memory device, which is integrated in a chip of semiconductor material. The memory device includes a plurality of memory cells. Each memory cell includes a first well and a second well of a first type of conductivity that are formed in an insulating region of a second type of conductivity. The memory cell further includes a first, a second, and a third region of the second type of conductivity that are formed in the first well; these regions define a selection transistor of MOS type and a storage transistor of floating gate MOS type that are coupled in series. Moreover, the memory device includes a selection gate of the selection transistor, a floating gate of the storage transistor, and a control gate of the storage transistor formed in the second well; the control gate is capacitively coupled with the floating gate. In an embodiment of the invention, the memory device further includes means for applying a first programming voltage to the first wells and a second programming voltage to the control gates of a subset of the memory cells including at least one memory cell to be programmed (with a difference between the first programming voltage and the second programming voltage that is capable of injecting electric charge into the floating gate of each memory cell to be programmed), for applying the first programming voltage to the third region of each memory cell to be programmed, and for applying a third programming voltage comprised between the first programming voltage and the second programming voltage to the third region of each memory cell of the subset not to be programmed (with a difference between the first programming voltage and the third programming voltage that inhibits the injection of electric charge into the corresponding floating gate).

A further embodiment is a system including one or more of these non-volatile memory devices.

Another embodiment is a method for programming this memory device.

Another embodiment is a corresponding method of integrating the non-volatile memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments, as well as features and advantages thereof, will be best understood with reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the accompanying drawings (wherein corresponding elements are denoted with equal or similar references and their explanation is not repeated for the sake of brevity, and the name of each entity is generally used to denote both its type and its attributes—such as its value, content and representation—for the sake of simplicity). In this respect, it is expressly intended that the figures are not necessary drawn to scale (with some details that may be exaggerated and/or simplified) and that, unless otherwise indicated, they are merely used to conceptually illustrate the structures and procedures described herein. Particularly:

FIG. 1 is a schematic block diagram of a non-volatile memory device wherein an embodiment is applicable.

FIG. 2A-FIG. 2B show a simplified implementation of a memory cell known in the art.

FIG. 2C is an equivalent circuit of the memory cell of the FIGS. 2A-2B.

FIG. 2D shows a conceptual representation of the architecture known in the art of the memory device based on this memory cell.

FIG. 2E-FIG. 2G show the biasing of the memory cell of the FIGS. 2A-2D during a programming operation, an erasing operation, and a reading operation, respectively, known in the art.

FIG. 3 is a cross-section view of a simplified implementation of another memory cell know in the art.

FIG. 4A-FIG. 4B show a simplified implementation of a memory cell according to an embodiment.

FIG. 4C is an equivalent circuit of an embodiment of the memory cell of FIGS. 4A-4B.

FIG. 4D shows a conceptual representation of the architecture according to an embodiment of the memory device based on this memory cell.

FIG. 4E-FIG. 4G show the biasing of this memory cell during a programming operation, an erasing operation, and a reading operation, respectively, in an embodiment.

FIG. 5 is a cross-section view of a simplified implementation of another memory cell according to an embodiment.

FIG. 6 is a schematic representation of the organization of a memory matrix according to a further embodiment.

FIG. 7 is a schematic representation of the organization of a memory matrix according to a still further embodiment.

DETAILED DESCRIPTION

With reference in particular to FIG. 1, there is shown a schematic block diagram of a non-volatile memory device 100 wherein an embodiment is applicable; particularly, the memory device 100 is of the Cost-Effective or FTP type. The memory device 100 includes a memory matrix 105, which is formed by a plurality of memory cells (not shown in the figure) being organized in rows and columns (for example, 128-512 rows and 512-1024 columns). Each memory cell stores a logic value (for example, one bit). For this purpose, the memory cell is based on a floating gate MOS transistor; this transistor has a threshold voltage that depends on an electric charge in its floating gate. The different levels of the threshold voltage represent corresponding logic values; conventionally, the memory cell is programmed (at a logic value 0) when it exhibits a high threshold voltage, whereas the memory cell is erased (at the logic value 1) when it exhibits a low threshold voltage.

As described in detail in the following, each memory cell may be read and programmed individually; typically, the memory device 100 simultaneously reads and programs a word that is stored into a corresponding page of memory cells in a same row of the memory matrix 105 (for example, 8 memory cells for a word of 8 bits). Conversely, the memory cells is erased at the level of sectors—for example, each sector consisting of a row of memory cells.

An address buffer 110 receives an address of a selected page in the memory matrix 105 (for a corresponding word). Particularly, a portion of the address is provided to a row decoder 115 r, which selects the row of the selected page in the memory matrix 105; the other portion of the address is instead supplied to a column decoder 115 c, which selects a column in the memory matrix 105 within each packet of columns associated with a corresponding i-th bit of all the words. The address buffer 110 is also used to select a sector that is to be erased (through the row decoder 115 r).

A read/write unit 120 controls operation of the row decoder 115 r and the column decoder 115 c. The read/write unit 120 also includes all the components (such as a power management unit with charge pumps, sense amplifiers, comparators, reference cells, pulse generators, and the like) that are used for writing (i.e., programming or erasing) the memory cells and for reading their logic values. The read/write unit 120 is also coupled with an input/output (I/O) buffer 125; the input/output buffer 125 receives a word to be written into the selected page of the memory matrix 105, or it provides a word that has been read from the selected page of the memory matrix 105 (through the column decoder 115 c).

Moving to FIG. 2A-FIG. 2B, there is shown a simplified implementation of a generic memory cell 205 known in the art (of the emitter-FTP type); particularly, FIG. 2A is a (partially cut away) top view illustrating a layout of the memory cell 205, whereas FIG. 2B is a cross-section view of the same memory cell 205 along the directrix A-A of FIG. 2A.

The whole memory device is integrated in a chip of semiconductor material; as usual, the concentrations of N type and P type impurities (or dopant) are denoted by adding the sign + or the sign − to the letters N and P to indicate a high or low concentration of impurities, respectively; the letters N and P without the addition of any sign + or − denote concentrations of intermediate value.

Particularly, the chip includes a P-substrate 210. A conductive pad (for example, made of metal) contacts the P-substrate 210 (for example, at a highly doped contact region thereof, not shown in the figure) through a window that is open in an insulating layer 215 (for example, a field oxide) protecting a front surface of the chip, so as to define a P-substrate terminal Tsub. All the memory cells 205 are formed in a common N-well 220 that extends in the P-substrate 210 from the front surface of the chip; for example, the N-well 220 is formed by a buried layer (being obtained with a high-energy implantation process) and an external contact ring. A conductive pad contacts the N-well 220 through a window of the insulating layer 215, so as to define an N-well terminal Tnw.

The memory cell 205 includes a storage section, which is housed in a storage P-well 225 that extends in the N-well 220 from the front surface of the chip (with the storage P-well 225 that is common for all the memory cells 205 of the corresponding row); a conductive pad contacts the storage P-well 225 through a window of the insulating layer 215, so as to define a storage P-well terminal Tspw. Particularly, the storage section of the memory cell 205 includes three N+regions 230, 235, and 240; all the N+regions 230, 235 and 240 extend in the storage P-well 225 from the front surface of the chip (for example, being obtained with a diffusion process), with the N+region 235 that is arranged between the N+region 230 and the N+region 240. A conductive pad contacts the N+region 230 through a window of the insulating layer 215 so as to define a corresponding terminal Ts, and another conductive pad contacts the N+region 240 through a window of the insulating layer 215 so as to define a corresponding terminal Td (while the N+region 235 is left floating). A polysilicon layer 245 extends over a channel region of the storage P-well 225 between the N+region 230 and the N+region 235; the polysilicon layer 245 is separated from the channel region of the storage P-well 225 by a thin portion of the insulating layer 215. A conductive pad contacts the polysilicon layer 245 through a window of the insulating layer 215 so as to define a corresponding terminal Tsg. An N+region 250 extends in the storage P-well 225 from the front surface of the chip; a conductive pad contacts the N+region 250 through a window of the insulating layer 215 so as to define a corresponding terminal Te.

The memory cell 205 further includes a control section, which is housed in a control P-well 255 that extends in the N-well 220 from the front surface of the chip (with the control P-well 255 that is common for all the memory cells 205 of the corresponding row); a conductive pad contacts the control P-well 255 through a window of the insulating layer 215, so as to define a control P-well terminal Tcpw. Particularly, the control section of the memory cell 205 includes an N+region 260, which extends in the control P-well 255 from the front surface of the chip (with the N+region 260 that is common for all the memory cells 205 of the corresponding row); a conductive pad contacts the N+region 260 through a window of the insulating layer 215 so as to define a corresponding terminal Tcg.

The storage section and the control section of the memory cell 205 are coupled through a polysilicon layer 265. The polysilicon layer 265 extends from a channel region of the storage P-well 225 (between the N+region 235 and the N+region 240) to the N+region 260. The polysilicon layer 265 is separated from the channel region of the storage P-well 225 and from the N+region 260 by a thin portion of the insulating layer 215, while it is separated from the rest of the chip by the whole (thicker) insulating layer 215.

An equivalent circuit of this memory cell 205 is shown in FIG. 2C. Considering FIG. 2C together with FIG. 2B, an N-MOS selection transistor Ms and an N-MOS floating gate storage transistor Mc are formed in a common body defined by the storage P-well 225. Particularly, the selection transistor Ms is defined by the N+region 230 (source), the N+region 235 (drain), and the polysilicon layer 245 (gate); the storage transistor Mc is instead defined by the N+region 235 (source), the N+region 240 (drain), the polysilicon layer 265 (floating gate), and the N+region 260 (which forms an implanted capacitor Ccg with the floating gate 265, so as to act as its control gate). The selection transistor Ms has a source terminal (Ts), a selection gate terminal (Tsg), and a drain terminal that is coupled to a source terminal of the storage transistor Mc, which in turn has a control gate terminal (Tcg) and a drain terminal (Td).

The memory cell 205 also includes a (composite) NPN injection transistor Bi. The injection transistor Bi is formed by two stray transistors. Particularly, a lateral stray transistor is defined by the N+region 250 (emitter), the storage P-well 225 (base), and the N+region 240 (collector); moreover, a vertical stray transistor is defined by the same N+region 250 (emitter), the same storage P-well 225 (base), and the N-well 220 (collector). The injection transistor Bi has an emitter terminal (Te), a base terminal (Tspw), and two collector terminals (Td and Tnw).

In the storage section of the memory cell 205, a diode Dspw is formed by the P-N junction between the storage P-well 225 and the N-well 220; the diode Dspw has an anode terminal (Tspw)—which is coupled to the base terminal of the injection transistor Bi and to the common body of the storage transistor Mc and the selection transistor Ms—and a cathode terminal (Tnw). Instead, in the control section of the memory cell 205 a diode Dcg is formed by the P-N junction between the control P-well 255 and the control gate region 260; the diode Dcg has an anode terminal (Tcpw) and a cathode terminal (Tcg), which is coupled to the capacitor Ccg. Moreover, as above a diode Dcpw is formed by the P-N junction between the control P-well 255 and the N-well 220; the diode Dcpw has an anode terminal (Tcpw) that is coupled to the cathode terminal of the diode Dcg, and a cathode terminal (Tnw). At the end, another diode Dsub is formed by the P-N junction between the P-substrate 210 and the N-well 220; the diode Dsub has an anode terminal (Tsub), and a cathode terminal (Tnw) that is coupled to the cathode terminals of the diodes Dspw and Dcpw.

A conceptual representation of the architecture know in the art of the memory device based on this memory cell 205 is shown in FIG. 2D. Considering FIG. 2D together with FIG. 2C, in the memory matrix (denoted with the reference 105 k) the common P-substrate terminal Tsub is biased to the lowest voltage that is available in the memory device (for example, −10V); in this way, the diode Dsub is always reverse biased, so as to insulate the N-well. The memory matrix 105 k then includes the common N-well terminal Tnw. Moreover, the source terminals Ts of all the memory cells 205 of the memory matrix 105 k are coupled together.

In each row of the memory matrix 105 k, the common storage P-well terminal Tspw and the common control P-well terminal Tcpw are coupled to a corresponding storage P-well line Wspw and control P-well line Wcpw, respectively; both the lines Wspw and Wcpw are driven by the row decoder (not shown in the figure). The control gate terminals Tcg of all the memory cells 205 of each row are coupled to a corresponding word line WL, which is driven by the row decoder. The drain terminals Td of all the memory cells 205 of each column of the memory matrix 105 k are coupled to a corresponding bit line BL, which is driven by the column decoder (not shown in the figure). The selection gate terminals Tsg and the emitter terminals Te of all the memory cells 205 of each row are coupled to a corresponding selection gate line Wsg and emitter line We, respectively (both of them driven by the row decoder).

As described in detail in the following, the storage transistor Mc is used to store the logic value of the memory cell 205. The selection transistor Ms is instead used to select the memory cell 205 to be read along the corresponding column (removing the effect of any unselected memory cell 205 on the same column that is depleted at a negative threshold voltage).

Particularly, with reference to FIG. 2E together with FIG. 2D, the memory device is programmed by Substrate Hot Electrons (SHE) injection. For this purpose, during a programming operation of a selected memory cell 205 (of a page in a corresponding row), the common source terminal Ts is left floating and all the selection gate lines Wsg are biased to a reference voltage (or ground). The bit line BL of the selected memory cell 205 (and then its drain terminal Td) is biased to a high positive voltage (for example, 5V), while the other bit lines BL are biased to ground. Moreover, the emitter line We of the selected memory cell 205 is biased to ground, while the other emitter lines We are biased to a low positive voltage (for example, 1V). The word line WL of the selected memory cell 205 (and then its control gate terminal Tcg) is biased to a very high positive voltage (for example, 6.5V), while the other word lines WL are biased to ground. All the control P-well lines Wcpw are biased to ground, and all the storage P-well lines Wspw are biased to the same voltage of 1V. The common N-well terminal Tnw is biased to the same voltage of 5V.

In this way, all the diodes Dspw, Dcpw and Dcg are reverse biased (so as to insulate the storage P-well, the control P-well and the control gate, respectively, of each row), and all the selection transistors Ms are switched off. The selected memory cell 205 is enabled by the voltage at its control gate terminal Tcg (6.5V), which is transferred to the body of the storage transistor Mc by capacitive coupling and then creates an N-channel therein by charge inversion, and by the voltage at its drain terminal Td (5V), which is directly applied to this N-channel. At the same time, the injection transistor Bi is switched on; therefore, substrate electrons are generated in the body of the storage transistor Mc, with part of them that diffuses towards its N-channel. The substrate electrons that reach the N-channel of the storage transistor Mc are accelerated by the electric field defined by the potential difference between the N-channel (5V) and the body (1V). Some of the substrate electrons (denoted as hot substrate electrons) gain energy higher than a potential barrier of the thin insulating layer below the floating gate, so as to be injected into it (and then remain trapped therein).

Conversely, in each other (unselected) memory cell 205 the biasing of its storage transistor Mc is unable to inject electrons into the corresponding floating gate; indeed, its control gate terminal Tcg and/or its drain terminal Td are at ground; in any case, the injection transistor Bi is always switched off.

As shown in FIG. 2F together with FIG. 2D, the memory device is instead erased by Fowler-Nordheim effect, at the level of sectors each one consisting of a row of memory cells 205. For this purpose, during an erasing operation of a selected sector, the common source terminal Ts, all the bit lines BL (and then all the drain terminals Td), and all the emitter lines We are left floating. The word line WL of the selected sector (and then the control gate terminals Tcg of its memory cells 205) is biased to a very high negative voltage (for example, −10V), while the other word lines WL are biased to ground. At the same time, the storage P-well line Wspw of the selected sector is biased to a high positive voltage (for example, 5V), while the other storage P-well lines Wspw are biased to ground. The selection gate lines Wsg are biased to the same voltages as the storage P-well lines Wspw 5V for the selected sector and ground otherwise). Moreover, all the control P-well lines Wcpw are biased to the same voltage of −10V, and the common N-well terminal Tnw is biased to the same voltage of 5V.

In this way, all the diodes Dspw, Dcpw and Dcg are reverse biased, and all the selection transistors Ms and the injection transistors Bi are switched off. Each memory cell 205 of the selected sector is enabled by the voltage at the control gate terminal Tcg of the storage transistor Mc (−10V), which is transferred to its floating gate by capacitive coupling, and by the voltage at its storage P-well (5V), which is directly applied to the body of the storage transistor Mc (with the voltage of 5V at the selection gate Tsg that reduces its electric stress); therefore, the very high electric field defined by the potential difference between the floating gate (−10V) and the body (5V) of the storage transistor Mc generates a low Fowler-Nordheim current by quantum-mechanical tunneling, which current extracts the electrons from its floating gate. This condition is maintained for a time sufficient to ensure that all the memory cells 205 of the selected sector are erased (for example, 150-300 μs). Therefore, because of the inevitable spread of characteristics of the memory cells 205, most of them will be over-erased to a negative threshold voltage (i.e., they are depleted).

Conversely, in each (unselected) memory cell 205 of the other sectors no electric field is defined between the floating gate and the body of the storage transistor Mc (since they are both at ground), so that no electrons are extracted from its floating gate. However, all the bit lines BL (floating) propagate the voltage of the storage P-well of the selected sector (5V) to the storage P-well of each unselected memory cell 205; therefore, the unselected memory cell 205 is subject to a electric stress (being caused by the electric field defined by the potential difference between the floating gate and the body). This electric stress may cause a loss of electrons in the floating gate of its storage transistor Mc after repeated erasing operations, with an undesired erasing of the unselected memory cell 205.

At the end, during a reading operation of a selected memory cell 205 (of a page in a corresponding row), as shown in FIG. 2G together with FIG. 2D, all the emitter lines We are left floating, and the common source terminal Ts is biased to ground. The bit line BL of the selected memory cell 205 (and then its drain terminal Td) is biased to a very low positive voltage (for example, 1V), while the other bit lines BL are biased to ground. At the same time, the word line WL of the selected memory cell 205 (and then its control gate terminal Tcg) is biased to a high positive voltage (for example, 5V), while the other word lines WL are biased to ground. The selection gate line Wsg of the selected memory cell 205 is biased to a low positive voltage (for example, 3V), while the other selection gate lines Wsg are biased to ground. All the control P-well lines Wcpw are biased to ground, all the storage P-well lines Wspw are biased to ground, and the common N-well terminal Tnw is biased to the same voltage of 5V.

In this way, all the diodes Dspw, Dcpw and Dcg are reverse biased, and all the injection transistors Bi are switched off. The selected memory cell 205 is enabled by the voltage at the control gate terminal Tcg of the storage transistor Mc (5V), which is transferred to its floating gate by capacitive coupling, and by the voltage at its drain (1V); at the same time, the selection transistor Ms is switched on (so as to bring the source of the storage transistor Mc to ground). If the selected memory cell 205 is erased it has a low threshold voltage, which is lower than the voltage that is applied between its floating gate (5V) and source Ts through the selection transistor Ms (0V); therefore, the storage transistor Mc is switched on and a current flows through the corresponding bit line BL, so that the logic value 1 is read. On the other hand, if the selected memory cell 205 is programmed it has a high threshold voltage (higher than the voltage that is applied between its floating gate and source); therefore, the storage transistor Mc is switched off and no current flows through the corresponding bit line BL, so that the logic value 0 is read.

Conversely, in each other (unselected) memory cell 205 that is coupled to the same bit line BL of the selected memory cell 205, the selection transistor Ms is switched off. In this way, no current can flow through the storage transistor Mc, even when the memory cell 205 is depleted (i.e., it exhibits a negative threshold voltage), so that the unselected memory cell 205 does not interfere with the reading of the selected memory cell 205. In each one of the other unselected memory cells 205, the storage transistor Mc is always switched off (and the corresponding bit line BL is disconnected from the read/write unit).

With reference now to FIG. 3, there is shown a cross-section view of a simplified implementation of another memory cell 305 know in the art. In this case, the control gate of the storage transistor Mc is implemented by means of a MOS transistor (instead of the implanted capacitor described above).

More specifically, the memory cell 305 is now formed in two distinct N-wells extending in the P-substrate 210 from the front surface of the chip—i.e., a storage N-well 320 s that includes the storage P-well 225 and a control N-well 320 c that includes the control P-well 255; a conductive pad contacts the storage N-well 320 s through a window of the insulating layer 215 so as to define a storage N-well terminal Tsnw, and another conductive pad contacts the control N-well 320 c through a window of the insulating layer 215 so as to define a control N-well terminal Tcnw. The control section of the memory cell 305 now includes two N+regions 360 a and 360 b, which extend in the control P-well 255 from the front surface of the chip at opposite sides of the polysilicon layer 265; both the N+regions 360 a and 360 b (or at least one of them) are contacted by the terminal Tcg, which is short-circuited to the control P-well 255 (and then to the control P-well terminal Tcpw).

In this case, in an equivalent circuit of the memory cell 305 a MOS capacitor Ccg′ is formed with the floating gate 265 by the control P-well 255, which then acts as control gate of the storage transistor Mc (again contacted by the control gate terminal Tcg short-circuited thereto). A diode Dsnw is now formed by the P-N junction between the P-substrate 210 and the storage N-well 320 s; the diode Dsnw has an anode terminal (Tsub) and a cathode terminal that is coupled to the cathode terminal of the diode Dspw. Likewise, a diode Dcnw is formed by the P-N junction between the P-substrate 210 and the control N-well 320 c; the diode Dcnw has an anode terminal (Tsub) and a cathode terminal that is coupled to the cathode terminal of the diode Dcpw.

The operation of the memory cell 305 is similar to the one described above. However, in this case is provided two distinct (storage and control) N-wells 320 s and 320 c (which are properly biased so as to reverse bias the corresponding diodes Dsnw and Dcnw). Indeed, since the control P-well 255 is now always at the same voltage of the control gate terminal Tcg, during the programming operation the control N-well 320 c is biased to a high positive voltage (i.e., at least 6.5V in the example at issue) to ensure that the diode Dcpw is reverse biased; for this purpose, the control N-well terminal Tcnw is coupled to a charge pump (capable of providing the required high voltage). This is instead not possible for the storage N-well 225; indeed, since the storage N-well 225 provides the high current required by the corresponding collector of the injection transistor Bi to program the memory cell 305, the N-well terminal Tsnw is coupled to a power supply terminal (providing a power supply voltage of the memory device).

Moving to FIG. 4A-FIG. 4B, there is shown a simplified implementation of a generic memory cell 405 according to an embodiment; as above, FIG. 4A is a (partially cut away) top view illustrating a layout of the memory cell 405, whereas FIG. 4B is a cross-section view of the same memory cell 405 along the directrix B-B of FIG. 4A.

The control section of the memory cell 405 is the same as the one of the above-described emitter-FTP memory cell known in the art. Conversely, in the storage section of the memory cell 405 the emitter region and its emitter terminal have been removed (see references 250 and Te in FIG. 2A).

An equivalent circuit of this memory cell 405 is shown in FIG. 4C. Considering FIG. 4C together with FIG. 4B, in this case—with respect to the above-described emitter-FTP memory cell—no injection transistor is present (see reference Bi in FIG. 2B).

A conceptual representation of the architecture according to an embodiment of the memory device based on this memory cell 405 is shown in FIG. 4D. Considering FIG. 4D together with FIG. 4C, in the memory matrix (denoted with the reference 105 i) the common P-substrate terminal Tsub is biased to ground (again consisting of the lowest voltage that is available in the memory device), so as to reverse bias the diode Dsub (and then insulate the N-well). However, in this case —with respect to the above-described emitter-FTP memory device—no emitter line is present (see reference We in FIG. 2D). Moreover, the source terminals Ts of all the memory cells 405 of each column of the memory matrix 105 i are coupled to the bit line BL of the preceding column (with the addition of a further bit line BL before the first column); for this purpose, each pair of adjacent memory cells 405 in the same row shares a common region that defines both the drain region of a left memory cell 405 of the pair and the source region of a right memory cell 405 of the pair. Moreover, two selection gate lines are now provided for each row: an odd selection gate line Wsgo is coupled to the selection gate terminals Tsg of all the memory cells 405 in the odd columns, while an even selection gate line Wsge is coupled to the selection gate terminals Tsg of all the memory cells 405 in the even columns.

As described in detail in the following, the selection transistor Ms (in addition to select the memory cell 405 to be read along the corresponding column) is now also used—by means of the two distinct odd and even selection gate lines Wsgo,Wsge—to select the memory cell 405 to be read in the corresponding pair coupled to the same bit line BL in the corresponding row.

Particularly, with reference to FIG. 4E together with FIG. 4D, the memory device is now programmed by Fowler-Nordheim effect. For this purpose, during a programming operation of a selected memory cell 405 (of a page in a corresponding row), the word line WL of the selected memory cell 405 (and then its control gate terminal Tcg) is biased to a very high positive voltage (for example, 13V), while the other word lines WL are biased to ground. At the same time, all the storage P-well lines Wspw are biased to ground. The bit line BL of the selected memory cell 405 is biased to ground, while the other bit lines BL are biased to a low positive voltage (for example, 3.5V); in this way, the drain terminals Td of the memory cells 405 in the column of the selected memory cell 405 (including its drain terminal Td) and the source terminals Ts of the memory cells 405 in the following column are brought to ground, while all the other drain terminals Td and source terminals Ts (including the source terminal Ts of the selected memory cell 405) are brought to 3.5V. Moreover, all the odd and even selection gate lines Wsgo,Wsge, and all the control P-well lines Wcpw are biased to ground; the common N-well terminal Tnw is instead biased to the same voltage of 13V.

In this way, all the diodes Dspw, Dcpw and Dcg are reverse biased, and all the selection transistors Ms are switched off. The selected memory cell 405 is enabled by the voltage at its control gate (13V), which is transferred to the floating gate of the storage transistor Mc by capacitive coupling, and by the voltage at the storage P-well (0V), which is directly applied to the body of the storage transistor Mc; in this case, the same voltage of 0V is also applied by the drain terminal Td to an N-channel being created by charge inversion in the body of the storage P-well under the floating gate, so that it does not affect its biasing. Therefore, the very high electric field defined by the potential difference between the floating gate (13V) and the body (0V) of the storage transistor Mc generates a Fowler-Nordheim current, which injects electrons into its floating gate (for a time sufficient to ensure that all the selected memory cells 405 are programmed—for example, 150-300 μs).

Conversely, each other (unselected) memory cell 405 of the same row of the selected memory cell 405 receives the same voltages of 13V at the control gate terminal Tcg and of 0V at the storage P-well terminal Tspw. However, the drain terminal Td now brings the N-channel of the storage transistor Mc to 3.5V. Therefore, the electric field defined by the potential difference between the floating gate (13V) and the body (3.5V) of the storage transistor Mc is reduced, and then unable to program the unselected memory cell 405. In each other unselected memory cell 405, both the control gate terminal Tcg and the storage P-well terminal Tspw are at ground, so that the biasing of the storage transistor Mc is unable to inject electrons into the corresponding floating gate.

As shown in FIG. 4F together with FIG. 4D, the memory device is again erased by Fowler-Nordheim effect at the sector level. For this purpose, during an erasing operation of a selected sector, all the bit lines BL (and then all the drain terminals Td and source terminals Ts) are left floating. The word line WL of the selected sector (and then the control gate terminals Tcg of its memory cells 405) is biased to ground, while the other word lines WL are biased to a very high positive voltage (for example, 13V); at the same time, all the storage P-well lines Wspw are biased to the same voltage of 13V. Moreover, all the odd and even selection gate lines Wsgo,Wsge are biased to the same voltage of 13V, all the control P-well lines Wcpw are biased to ground, and the common N-well terminal Tnw is biased to the same voltage of 13V.

In this way, all the diodes Dspw, Dcpw and Dcg are reverse biased, and all the selection transistors Ms are switched off. Each memory cell 405 of the selected sector is enabled by the voltage at its control gate (0V), which is transferred to the floating gate of the storage transistor Mc by capacitive coupling, and by the voltage at the storage P-well (13V), which is directly applied to the body of the storage transistor Mc (with the voltage of 13V at the selection gate Tsg that reduces the electric stress on the selection transistor Ms). Therefore, the very high electric field defined by the potential difference between the floating gate (0V) and the body (13V) of the storage transistor Mc generates a Fowler-Nordheim current, which now extracts the electrons from its floating gate (for a time sufficient to ensure that all the memory cells 405 of the selected sector are erased). It should be noted that in this case it is not possible to use the bit lines BL to reduce the potential difference between the floating gate and the body of each storage transistor Mc selectively (like in the programming operation described above), since no N-channel is now formed in the body of the storage transistor Mc so that any voltage at the drain terminal Td would be unable to change its biasing.

Conversely, in each (unselected) memory cell 405 of the other sectors no electric field is defined between the floating gate and the body of the storage transistor Mc (since they are both at 13V), so that no electrons are extracted from its floating gate.

At the end, during a reading operation of a selected memory cell 405 (of a page in a corresponding row), as shown in FIG. 4G together with FIG. 4D, the word line WL of the selected memory cell 405 (and then its control gate terminal Tcg) is biased to a high positive voltage (for example, 5V), while the other word lines WL are biased to ground. At the same time, the bit line BL of the selected memory cell 405 is biased to a very low positive voltage (for example, 1V), while the other bit lines BL are biased to ground; in this way, the drain terminals Td of the memory cells 405 in the column of the selected memory cell 405 (including its drain terminal Td) and the source terminals Ts of the memory cells 405 in the following column are brought to 1V, while all the other drain terminals Td and source terminals Ts (including the source terminal Ts of the selected memory cell 405) are brought to ground. Moreover, the odd or even selection gate line Wsgo,Wsge of the selected memory cell 405 is biased to the same voltage of 3V, while the other odd and even selection gate lines Wsgo,Wsge (in the same row and in the other rows) are biased to ground. All the storage P-well lines Wspw and all the control P-well lines Wcpw are biased to ground, and the common N-well terminal Tnw is biased to the same voltage of 5V.

In this way, all the diodes Dspw, Dcpw and Dcg are reverse biased. Each selected memory cell 405 is enabled by the voltage at the gate terminal Tcg of the storage transistor Mc (5V), which is transferred to its floating gate by capacitive coupling, and by the voltage at its drain terminal Td (1V); at the same time, the selection transistor Ms is switched on, so as to bring the source of the storage transistor Mc to ground. As above, if the selected memory cell 405 is erased its storage transistor Mc (with low threshold voltage) is switched on by the potential difference between the floating gate (5V) and the source (0V), and a current flows through the corresponding bit line BL so that the logic value 1 is read. On the other hand, if the selected memory cell 405 is programmed its storage transistor Mc (with high threshold voltage) is switched off and no current flows through the corresponding bit line BL, so that the logic value 0 is read.

Conversely, in the next (unselected) memory cell 405 in the same row of the selected memory cell 405 (whose source terminal Ts is coupled to the same bit line BL), the selection transistor Ms is switched off; likewise, in each unselected memory cell 405 of the same column of the selected memory cell 405 the selection transistor Ms is switched off. Therefore, no current can flow through the storage transistor Mc of each one of these unselected memory cells 405, even when they are depleted. In each one of the other unselected memory cells 405, the storage transistor Mc is always switched off (and the corresponding bit line BL is disconnected from the read/write unit).

Naturally, in this embodiment it is not possible to read the memory cells 405 on the same row that share the same bit line BL concurrently; therefore, all the memory cells 405 of each page are coupled to the same (odd or even) selection gate line Wsgo,Wsge.

An embodiment greatly reduces the currents that are used during the programming operation (since it in now implemented by Fowler-Nordheim effect). As a result, the charge pumps of the memory device may be simplified, with a corresponding reduction of their size. Moreover, the low currents that are used during the programming operation accordingly reduce the power consumption of the memory device. In this way, it may also be possible to have a higher programming parallelism of the memory device (for example, 16-32 bits).

The programming of the memory cells by Fowler-Nordheim effect may be slower than their programming by SHE injection (as in the emitter-FTP memory cells known in the art); however, this may be compensated for by the higher programming parallelism of the proposed memory device, so that its overall programming speed remains comparable to the programming speed of the emitter-FTP memory devices.

An embodiment may also allow a reduction in the size of each memory cell (since it removes the emitter region of the injection transistor); therefore, this may allow producing memory devices with higher capacity (for example, up to some hundreds of Kbytes).

Particularly, in the above-described embodiment all the voltages that are used during the programming, erasing, and reading operations are positive. Therefore, the required high voltages may be generated (from the power supply voltage) by means of a single (positive) charge pump. This charge pump (which provides all the high voltages with low current that are required during both the programming operation and the erasing operation—i.e., up to 13V) may be larger than each one of the charge pumps that are used in the emitter-FTP memory device —i.e., a positive charge pump (which provides the positive high voltages with low currents that are required during the programming operation—i.e., up to 6.5V), another positive charge pump (which provides the positive high voltages with high currents that are required during the programming operation—i.e., up to 5V), and a negative charge pump (which provides the negative high voltages with low currents that are required during the erasing operation—i.e., up to −10V); in any case, the charge pump of an embodiment of the memory device is smaller than the sum of the three charge pumps (and even of the sum of the two charge pumps with low current) of the emitter-FTP memory device.

It is noted that, despite the operation of the memory device requiring higher voltages in absolute value, the voltages that are applied between each pair of terminals of the selection transistor Ms never exceed the power supply voltage (i.e., 3.5V); therefore, the selection transistor Ms may still be implemented with a (standard) low voltage component.

Moreover, during the programming operation the control gate and the body (i.e., the storage P-well) of each unselected memory cell 405 (not included in the row of the selected memory cells 405) are at the same voltage of 0V; likewise, during the erasing operation the control gate and the body (i.e., the N-well) of each unselected memory cell 405 are at the same voltage of 13V. This may greatly reduce the electric stress on the unselected memory cells (and, therefore, reduces the increase of electrons in the floating gates of their storage transistors during the programming operation and reduces the loss of electrons from the floating gates of their storage transistors during the erasing operation), with a beneficial effect on the data retention of the memory device. In this way, an embodiment of the memory device may withstand a higher number of programming/erasing operations—being comparable to the ones of the E²PROMs (for example, of the order of 100,000-200,000).

With reference now to FIG. 5, there is shown a cross-section view of a simplified implementation of another memory cell 505 according to an embodiment. As above, the control gate of the storage transistor Mc is now implemented by means of the MOS transistor being formed by the N+regions 360 a and 360 b, with the capacitor Ccg′ that is directly coupled to the anode of the diode Dcpw.

Nevertheless, in this case it still may be possible to form all the memory cells in the common N-well 220. Indeed, during the programming and erasing operations the N-well 220 may now be biased to the high positive voltage (i.e., 13V in the example at issue) to ensure that the diode Dsub is reverse biased, since no current flows through the N-well 220 and then the N-well terminal Tnw may be coupled to a charge pump providing this voltage.

Moving to FIG. 6, there is shown a schematic representation of the organization of the memory matrix (denoted with the reference 105 i′) according to a further embodiment. Particularly, the memory cells (based on the implanted capacitors, wherein the control gates are insulated from the control P-wells) are now grouped in pairs of adjacent rows. The memory cells of each pair of rows (differentiated with the addition of a suffix “a” and “b”, respectively) are symmetric with respect to a row direction (i.e., horizontally in the figure). The memory cells 405 a, 405 b of the pair of rows then share a common control P-well with the corresponding terminal (denoted with 255′ and Tcpw′, respectively)—wherein both the N+region 260 a and the N+region 260 b are formed. Indeed, the control P-well 255′ is always biased to the same voltage during the different operations on the memory device (i.e., programming, erasing and reading) for both the selected memory cells 405 a, 405 b and the unselected memory cells 405 a, 405 b (see FIG. 4E-FIG. 4G). Therefore, the control sections of all the memory cells of the memory matrix 105 i′ might be formed in a common control P-well; however, because of routing problems, it may be preferable to provide a distinct control P-well 255′ for the memory cells 405 a, 450 b of each pair of rows (with all the control P-well terminals Tcpw′ of the memory matrix 105 i′ that may be coupled together).

The above-described structure of the memory matrix 105 i′ further reduces the size of the memory device as a whole.

Considering now FIG. 7, there is shown a schematic representation of the organization of the memory matrix (denoted with the reference 105 i″) according to a still further embodiment. Particularly, the memory cells (based on the implanted capacitors in the example at issue, but with similar considerations that may apply to the memory cells based on the MOS capacitors) are again grouped in pairs of adjacent rows; as above, the memory cells of each pair of rows (differentiated with the addition of a suffix “c” and “d”, respectively) are symmetric with respect to the row direction. However, the memory cells 405 c, 405 d of the pair of rows now share a common storage P-well with the corresponding terminal (denoted with 225″ and Tspw″, respectively)—wherein all the N+regions 230 c, 235 c, 240 c, 230 d, 235 d and 240 d are formed. Indeed, the storage P-well 225″ is always biased to the same voltage during the different operations on the memory device (i.e., programming, erasing and reading) for both the selected memory cells 405 c, 405 d and the unselected memory cells 405 c, 405 d (see FIG. 4E-FIG. 4G). Therefore, the storage sections of all the memory cells of the memory matrix 105 i″ might be formed in a common storage P-well; however, because of routing problems, it may preferable to provide a distinct storage P-well 225″ for the memory cells 405 c, 450 d of each pair of rows (with all the storage P-well terminals Tspw″ of the memory matrix 105 i″ that may be coupled together).

As above, this structure of the memory matrix 105 i″ may further reduce the size of the memory device as a whole. Moreover, an additional reduction of the size of the memory device may be obtained when the embodiment of FIG. 7 (i.e., with a single control P-well for each pair of rows) is combined with the embodiment of FIG. 6 (i.e., with a single storage P-well for each pair of rows). Particularly, in this case the memory matrix would include common storage P-wells (each one for the storage sections of a corresponding pair of rows) being alternated with common control P-wells (each one for the control sections of a corresponding pair of rows); the upper storage sections of each storage P-well are associated with the lower control sections of the control P-well above it, while the lower storage sections of each storage P-well are associated with the upper control sections of the control P-well below it.

Naturally, in order to satisfy local and specific requirements, one may apply to the embodiments described above many logical and/or physical modifications and alterations. More specifically, although this disclosure has been described with a certain degree of particularity with reference to one or more embodiments thereof, it is understood that omissions, substitutions and changes in the form and details as well as other embodiments are possible. Particularly, different embodiments may even be practiced without the specific details (such as the numerical examples) set forth in the preceding description to provide a more thorough understanding thereof; conversely, well-known features may have been omitted or simplified in order not to obscure the description with unnecessary particulars. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any embodiment of the disclosed solution may be incorporated in any other embodiment as a matter of general design choice.

For example, similar considerations apply if the memory device has a different structure or includes equivalent components (either separate from each other or combined together, in whole or in part); moreover, the memory device may have different operative characteristics.

In addition, the regions of the N-type may be replaced by regions of the P-type, and vice-versa (so as to obtain a memory device based on transistors of the P-MOS type); particularly, when the substrate remains of the P-type, it is also possible to form the storage wells and the control wells (of the N-type) directly in the substrate without the above-described triple-well structure. Alternatively, each memory cell may store a different logic value (for example, two or more bits).

Voltages of any other value may be used to cause the injection of electric charge (i.e., electrons for N-MOS transistors and holes for P-MOS transistors) into the floating gates of the memory cells to be programmed, and to inhibit it in the other memory cells.

The above-mentioned layout embodiments are merely illustrative and in no way limitative (with the different regions of each memory cell that may be arranged in any other way). Similar considerations may apply to the architecture of the memory device. For example, in order to increase the reliability of the memory device, each logic value may be stored in a pair of memory cells (associated with the same address): a direct cell stores the actual logic value, and a complementary cell stores its opposite value. During a reading operation, both the direct cell and the complementary cell are read. If the direct cell is programmed and the complementary cell is erased, the logic value 0 is read; conversely, if the direct cell is erased and the complementary cell is programmed, the logic value 1 is read. Instead, when the direct cell and the complementary cell are both programmed or both erased, an error condition is reported.

Naturally, the rows and columns of the memory matrix are merely topological concepts, which are not bound to any specific geometric arrangement.

A basic implementation wherein no bit line is shared between adjacent columns of memory cells is feasible (with the addition of a further bit line for the source terminals of the memory cells of each column).

The above-mentioned voltages being used during the programming, erasing and reading operations are merely illustrative and they are not to be interpreted in a limitative manner. Indeed, the voltages may vary according to the structure of the memory device (and particularly a thickness of its insulating layer); moreover, dual considerations apply if the memory device is implemented with P-MOS transistors. More generally, the programming voltages and the erasing voltages may differ from each other.

Naturally, when the memory device is implemented with P-MOS transistors the biasing voltages (being used during the programming, erasing and reading operation) may be all negative; in any case, the use of biasing voltages in part positive and in part negative is not excluded.

Nothing prevents providing multiple N-wells (for example, each one for a predefined subset of rows of memory cells).

The possibility of forming the control sections of a different number of memory cells (based on implanted capacitors) in the same control P-well is not excluded.

Likewise, the possibility of forming the storage sections of a different number of memory cells in the same storage P-well is not excluded.

Similar considerations may apply if the memory device is programmed, erased and/or read with equivalent procedures (by using similar steps, removing some steps being non-essential, or adding further optional steps).

The memory device may be produced with other production processes (for example, of the smart power, analog, or high voltage type). It is understood that an embodiment might be part of the design of an integrated circuit. The design may also be created in a programming language; moreover, if the designer does not fabricate chips or masks, the design may be transmitted by physical means to others. In any case, the resulting integrated circuit may be distributed by its manufacturer in raw wafer form, as a bare die, or in packages.

Moreover, an embodiment of such an integrated circuit may be integrated with other circuits in the same chip; the chip may also be coupled with one or more other chips (such as a processor), or it may be mounted in intermediate products (such as mother boards). In any case, an embodiment of a memory device is suitable to be used in complex systems (such as mobile telephone).

From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated. 

1. A non-volatile memory device integrated in a chip of semiconductor material, the memory device including a plurality of memory cells each one including a first well and a second well of a first type of conductivity being formed in an insulating region of a second type of conductivity, a first, a second, and a third region of the second type of conductivity being formed in the first well for defining a selection transistor of MOS type and a storage transistor of floating gate MOS type being coupled in series, a selection gate of the selection transistor, a floating gate of the storage transistor, and a control gate of the storage transistor formed in the second well, the control gate being capacitively coupled with the floating gate, means for applying a first programming voltage to the first wells and a second programming voltage to the control gates of a subset of the memory cells including at least one memory cell to be programmed, a difference between the first programming voltage and the second programming voltage being capable of injecting electric charge into the floating gate of each memory cell to be programmed, for applying the first programming voltage to the third region of each memory cell to be programmed, and for applying a third programming voltage comprised between the first programming voltage and the second programming voltage to the third region of each memory cell of the subset not to be programmed, a difference between the first programming voltage and the third programming voltage inhibiting the injection of electric charge into the corresponding floating gate.
 2. The memory device according to claim 1, wherein the memory cells are arranged in a matrix with a plurality of rows and a plurality of columns, the memory cells of each row sharing a common first well and a common second well, and wherein the memory device further includes a plurality of first well lines each one coupled to the first well of a corresponding row, a plurality of second well lines each one coupled to the second well of a corresponding row, a plurality of word lines each one coupled to the control gates of the memory cells of a corresponding row, a plurality of sets of selection lines each one coupled to the selections gates of the memory cells of a corresponding row, and a plurality of bit lines each one coupled to the third regions of the memory cells of a corresponding line.
 3. The memory device according to claim 2, wherein each memory cell of each row different from a first cell of the row shares the first region of the memory cell with the third region of a preceding memory cell in the row, each set of selection lines including an odd selection line and an even selection lines each one coupled alternatively to the selection gates of the memory cells of the corresponding row, the selection gates of each pair of adjacent memory cells being coupled to different odd and even selection lines.
 4. The memory device according to claim 3, further including programming means, during a programming operation of a set of selected memory cells of a common row the programming means applying the first programming voltage to all the first well lines, applying the second programming voltage to the word line of the selected memory cells and the first programming voltage to the other word lines, applying the first programming voltage to the bit lines of the selected memory cells and the third programming voltage to the other bit lines, and applying the first programming voltage to all the selection lines.
 5. The memory device according to claim 4, further including erasing means, during an erasing operation of all the memory cells of a selected row the erasing means leaving all the bit lines floating, applying a first erasing voltage to the word line of the selected row and a second erasing voltage to the other word lines, applying the second erasing voltage to all the first well lines, and applying the second erasing voltage to all the selection lines.
 6. The memory device according to claim 5, further including reading means, during a reading operation of a set of further selected memory cells of a common row the reading means applying a first reading voltage to all the first well lines, applying a second reading voltage to the word line of the further selected memory cells and the first reading voltage to the other word lines, applying a third reading voltage comprised between the first reading voltage and the second reading voltage to the bit lines of the further selected memory cells and the first reading voltage to the other bit lines, and biasing the selection lines to switch on the selection transistor of each further selected memory cell and to switch off the selection transistor of the other memory cells.
 7. The memory device according to claim 6, further including means for generating the programming voltages, the erasing voltages and the reading voltages from a reference voltage and a power supply voltage of the memory device, all the programming voltages, erasing voltages and reading voltages having a same sign as the power supply voltage with respect to the reference voltage.
 8. The memory device according to claim 1, wherein the rows of the matrix are grouped in pairs of adjacent rows sharing a common first well.
 9. The memory device according to claim 2, wherein each memory cell includes a fourth region of the second type of conductivity formed in the second well in front of the floating gate for defining an implanted capacitor, the forth region forming the control gate of the storage transistor, and wherein the rows of the matrix are grouped in pairs of adjacent rows sharing a common second well.
 10. The memory device according to claim 1, wherein each memory cell includes a fourth region and a fifth region of the second type of conductivity formed in the second well with a channel region therebetween in front of the floating gate for defining a control transistor of MOS type, the channel region being short-circuited to at least one of the fourth and fifth region to form the control gate of the storage transistor, and wherein all the memory cells of the matrix share a common insulating region.
 11. A non-volatile memory cell, comprising: an access transistor having a gate, a body, and first and second conduction nodes; and a storage transistor having a floating gate, a body coupled to the body of the access transistor, a first conduction node coupled to the second conduction node of the access transistor, and a second conduction node, the storage transistor operable to be programmed by electrons that flow from the body of the storage transistor into the floating gate via tunnelling.
 12. The non-volatile memory cell of claim 11 wherein the storage and access transistors comprise respective N-channel transistors
 13. The non-volatile memory cell of claim 11, further comprising a word-line node capacitively coupled to the floating gate of the storage transistor.
 14. The non-volatile memory cell of claim 11 wherein the storage transistor is operable to be erased by electrons that flow from the floating gate to the body of the storage transistor via tunnelling.
 15. The non-volatile memory cell of claim 11 wherein the storage transistor is operable to be programmed by electrons that flow from the body of the storage transistor to the floating gate via Fowler-Nordheim tunnelling.
 16. The non-volatile memory cell of claim 11, further comprising an access-control node coupled to the gate of the access transistor.
 17. The non-volatile memory cell of claim 11, further comprising a bit-line node coupled to the first conduction node of the access transistor.
 18. The non-volatile memory cell of claim 11, further comprising a bit-line node coupled to the second conduction node of the storage transistor.
 19. The non-volatile memory cell of claim 11, further comprising: a first word-line node coupled to the floating gate of the storage transistor; and a second word-line node coupled to the gate of the access transistor.
 20. A memory array, comprising: first and second bit lines; a first access-control line; a first word line; a first body line; and a first non-volatile memory cell, comprising: a first access transistor having a gate coupled to the first access-control line, a body coupled to the first body line, a first conduction node coupled to the first bit line, and a second conduction node; and a first storage transistor having a floating gate coupled to the first word line, a body coupled to the first body line, a first conduction node coupled to the second conduction node of the access transistor, and a second conduction node coupled to the second bit line; and a programming circuit operable to program the storage transistor by causing electrons to flow from the body of the storage transistor into the floating gate via tunnelling.
 21. The memory array of claim 20 wherein the programming circuit is operable to program the storage transistor by: generating a first voltage on the first bit line; generating a second voltage on the second bit line, the second voltage being lower in magnitude than the first voltage; generating a third voltage on the first word line, the third voltage being higher in magnitude than the first voltage; generating a fourth voltage on the first access-control line, the fourth voltage being lower in magnitude than the first voltage; and generating a fifth voltage on the first body line, the fifth voltage being lower in magnitude than the first voltage.
 22. The memory array of claim 20, further comprising an erasing circuit operable to erase the storage transistor by causing electrons to flow from the floating gate into the body of the storage transistor via tunnelling.
 23. The memory array of claim 20, further comprising an erasing circuit operable to erase the storage transistor by: causing the first and second bit lines to float electrically; generating a first voltage on the first word line; generating a second voltage on the first access-control line, the second voltage being higher in magnitude than the first voltage; and generating a third voltage on the body line, the third voltage being approximately equal to the second voltage.
 24. The memory array of claim 20, further comprising: a second word line; a second access-control line; a second body line; a second non-volatile memory cell, comprising: a second access transistor having a gate coupled to the second access-control line, a body coupled to the second body line, a first conduction node coupled to the first bit line, and a second conduction node; and a second storage transistor having a floating gate coupled to the second word line, a body coupled to the second body line, a first conduction node coupled to the second conduction node of the access transistor, and a second conduction node coupled to the second bit line; and wherein the programming circuit is operable to program the first storage transistor by: generating a first voltage on the first bit line; generating a second voltage on the second bit line, the second voltage being lower in magnitude than the first voltage; generating a third voltage on the first word line, the third voltage being higher in magnitude than the first voltage; generating a fourth voltage on the second word line, the fourth voltage being lower in magnitude than the third voltage; generating respective fourth and fifth voltages on the first and second access-control lines, the fourth and fifth voltages being lower in magnitude than the first voltage; and generating respective sixth and seventh voltages on the first and second body lines, the sixth and seventh voltages being lower in magnitude than the first voltage.
 25. The memory array of claim 20, further comprising: a second word line; a second access-control line; a second body line; a second non-volatile memory cell, comprising: a second access transistor having a gate coupled to the second access-control line, a body coupled to the second body line, a first conduction node coupled to the first bit line, and a second conduction node; and a second storage transistor having a floating gate coupled to the second word line, a body coupled to the second body line, a first conduction node coupled to the second conduction node of the access transistor, and a second conduction node coupled to the second bit line; and an erasing circuit operable to erase the first storage transistor by: causing the first and second bit lines to float electrically; generating a first voltage on the first word line; generating a second voltage on the second word line, the second voltage being higher in magnitude than the first voltage; generating respective fourth and fifth voltages on the first and second access-control lines, the fourth and fifth voltages being approximately equal to the second voltage; and generating respective sixth and seventh voltages on the first and second body lines, the sixth and seventh voltages being approximately equal to the second voltage.
 26. The memory array of claim 20, further comprising: a third bit line; a second access-control line; a second non-volatile memory cell, comprising: a second access transistor having a gate coupled to the second access-control line, a body coupled to the first body line, a first conduction node coupled to the second bit line, and a second conduction node; and a second storage transistor having a floating gate coupled to the first word line, a body coupled to the first body line, a first conduction node coupled to the second conduction node of the second access transistor, and a second conduction node coupled to the third bit line; and wherein the programming circuit is operable to program the first storage transistor by: generating a first voltage on the first and third bit lines; generating a second voltage on the second bit line, the second voltage being lower in magnitude than the first voltage; generating a third voltage on the first word line, the third voltage being higher in magnitude than the first voltage; generating respective fourth and fifth voltages on the first and second access-control lines, the fourth and fifth voltages being lower in magnitude than the first voltage; and generating a sixth voltage on the first body line, the sixth voltage being lower in magnitude than the first voltage.
 27. The memory array of claim 20, further comprising: a third bit line; a second access-control line; a second non-volatile memory cell, comprising: a second access transistor having a gate coupled to the second access-control line, a body coupled to the first body line, a first conduction node coupled to the second bit line, and a second conduction node; and a second storage transistor having a floating gate coupled to the first word line, a body coupled to the first body line, a first conduction node coupled to the second conduction node of the second access transistor, and a second conduction node coupled to the third bit line; and an erasing circuit operable to erase the first storage transistor by: causing the first, second, and third bit lines to float electrically; generating a first voltage on the first word line; generating respective second and third voltages on the first and second access-control lines, the second and third voltages being higher in magnitude than the first voltage; and generating a fourth voltage on the first body line, the fourth voltage being higher in magnitude than the first voltage.
 28. The memory array of claim 20, further comprising: a third bit line; a second access-control line; a second non-volatile memory cell, comprising: a second access transistor having a gate coupled to the second access-control line, a body coupled to the first body line, a first conduction node coupled to the third bit line, and a second conduction node; and a second storage transistor having a floating gate coupled to the first word line, a body coupled to the first body line, a first conduction node coupled to the second conduction node of the second access transistor, and a second conduction node coupled to the first bit line; and wherein the programming circuit is operable to program the first storage transistor by: generating a first voltage on the first and third bit lines; generating a second voltage on the second bit line, the second voltage being lower in magnitude than the first voltage; generating a third voltage on the first word line, the third voltage being higher in magnitude than the first voltage; generating respective fourth and fifth voltages on the first and second access-control lines, the fourth and fifth voltages being lower in magnitude than the first voltage; and generating a sixth voltage on the first body line, the sixth voltage being lower in magnitude than the first voltage.
 29. The memory array of claim 20, further comprising: a third bit line; a second access-control line; a second non-volatile memory cell, comprising: a second access transistor having a gate coupled to the second access-control line, a body coupled to the first body line, a first conduction node coupled to the third bit line, and a second conduction node; and a second storage transistor having a floating gate coupled to the first word line, a body coupled to the first body line, a first conduction node coupled to the second conduction node of the second access transistor, and a second conduction node coupled to the first bit line; and an erasing circuit operable to erase the first storage transistor by: causing the first, second, and third bit lines to float electrically; generating a first voltage on the first word line; generating respective second and third voltages on the first and second access-control lines, the second and third voltages being higher in magnitude than the first voltage; and generating a fourth voltage on the first body line, the fourth voltage being higher in magnitude than the first voltage.
 30. A system, comprising: an integrated memory array, comprising: first and second bit lines; a first access-control line; a first word line; a first body line; and a first non-volatile memory cell, comprising: a first access transistor having a gate coupled to the first access-control line, a body coupled to the first body line, a first conduction node coupled to the first bit line, and a second conduction node; and a first storage transistor having a floating gate coupled to the first word line, a body coupled to the first body line, a first conduction node coupled to the second conduction node of the access transistor, and a second conduction node coupled to the second bit line; and a programming circuit operable to program the storage transistor by causing electrons to flow from the body of the storage transistor into the floating gate via tunnelling; and an integrated circuit coupled to the integrated memory array.
 31. The system of claim 30 wherein the integrated memory array and the integrated circuit are disposed on a same integrated-circuit die.
 32. The system of claim 30 wherein the integrated memory array and the integrated circuit are disposed on respective integrated-circuit dies.
 33. The system of claim 30 wherein the integrated circuit comprises a controller.
 34. A method, comprising: electrically isolating a first conduction node of a first storage transistor with a first access transistor; coupling a floating gate of the storage transistor to a first voltage; coupling a second conduction node of the storage transistor to a second voltage; and coupling a body of the storage transistor to approximately the second voltage to inject a level of charge onto the floating gate via tunnelling through an insulator.
 35. The method of claim 34 wherein electrically isolating the first conduction node comprises electrically isolating the first conduction node of the storage transistor from a bit line with the access transistor.
 36. The method of claim 34 wherein the first voltage is higher than the second voltage.
 37. The method of claim 34, further comprising: coupling the floating gate of the storage transistor to a third voltage; allowing the second conduction node of the storage transistor to float electrically; and coupling the body of the storage transistor to a fourth voltage that is higher in magnitude than the third voltage to remove a level of charge from the floating gate via tunnelling through an insulator.
 38. The method of claim 34, further comprising while coupling the body of the first storage transistor to the second voltage: electrically isolating a first conduction node of a second storage transistor with a second access transistor; coupling a floating gate of the second storage transistor to a third voltage that is less than the first voltage; coupling the second conduction node of the second storage transistor to the second voltage; and coupling the body of the second storage transistor to approximately the second voltage to maintain approximately unchanging a level of charge on the floating gate of the second storage transistor.
 39. The method of claim 34, further comprising while coupling the body of the first storage transistor to the second voltage: electrically isolating a first conduction node of a second storage transistor with a second access transistor; coupling a second conduction node of the second storage transistor to a third voltage having a magnitude less than a magnitude of the first voltage and greater than a magnitude of the second voltage to maintain approximately unchanging a level of charge on a floating gate of the second storage transistor; coupling a floating gate of the second storage transistor to the first voltage; and coupling the body of the second storage transistor to approximately the second voltage.
 40. The method of claim 34, further comprising: coupling the floating gate of the first storage transistor to a third voltage; allowing the second conduction node of the storage transistor to float electrically; coupling the body of the first storage transistor to a fourth voltage that is higher in magnitude than the third voltage to remove a level of charge from the floating gate via tunnelling through an insulator; coupling the floating gate of a second storage transistor to approximately the fourth voltage to maintain approximately unchanging a level of charge on a floating gate of the second storage transistor; and allowing a second conduction node of the second storage transistor to float electrically.
 41. The method of claim 34 wherein coupling the floating gate of the storage transistor comprises capacitively coupling the floating gate to the first voltage.
 42. The method of claim 34, further comprising: coupling the floating gate of the first storage transistor to a third voltage; allowing the second conduction node of the storage transistor to float electrically; coupling the body of the first storage transistor to a fourth voltage that is higher in magnitude than the third voltage to remove a level of charge from the floating gate via tunnelling through an insulator; coupling the floating gate of a second storage transistor to approximately the fourth voltage to maintain approximately unchanging a level of charge on a floating gate of the second storage transistor; and allowing a second conduction node of the second storage transistor to float electrically. 